QC Calculations

Quantum Chemical Calculations

pDynamo incorporates both ab initio density functional
theory (DFT) and semi-empirical QC methods. The usage of both types of
method is similar and they can be employed interchangeably. Example5.py is a program that calculates the
potential energy, atomic charges and dipole moment for a molecule with different semi-empirical QC methods. It is:

The program starts by defining the list of different QC models
to use and then loops over them, repeating an identical set of
calculations on a water molecule for each model. It is important to note
that the QC model is assigned to the system, via the method DefineQCModel of the System
class, before determination of the energy and other properties occurs.
The program terminates by printing out a table containing the calculated
results.

Exercises

Modify the example program to use DFT methods in addition to the semi-empirical ones. See the DFT QC Models tutorial for full details of how to do this. How much longer do DFT calculations take to run compared to semi-empirical ones?

Carry out calculations for different charge and multiplicity
states of a molecular system. How reliable are the calculations?

Compute the dissociation curve for a system (e.g. dihydrogen or
ethane) as a function of a bond length. How do the results of
spin-restricted and spin-unrestricted calculations compare?